1. The Field of the Invention
The present invention relates to systems and methods for stabilizing a portion of a spine. More specifically, the invention relates to anchors and related components that are selectively mounted on a spine for stabilizing the spine.
2. The Relevant Technology
There are many surgical procedures and treatments that require the immobilization of a portion of the spine. For example, vertebral fusion is a medical procedure where adjacent vertebrae of the spine are fused together. As part of this procedure, a mechanical stabilizing system is implanted in the patient which immobilizes the adjacent vertebrae. Such stabilizing systems can also be used in the treatment of spinal trauma and spinal curvature such as scoliosis.
A typical spinal stabilizing system includes, in part, a pair of anchors and a rigid rod that extends between the anchors. The anchors are fixed to the adjacent vertebrae such that when the rod is connected to the anchors, the adjacent vertebrae become immobilized. A typical anchor includes a cylindrical tubular body having a longitudinal passage extending therethrough and a transverse passage extending therethrough. The exterior surface of the tubular body is round and has threads thereon to receive a nut.
The anchor also includes an elongated screw having an enlarged head formed on one end thereof. The head has a polygonal socket formed thereon in alignment with the longitudinal axis of the screw. The enlarged head of the screw is seated within the tubular body such that the tubular body can freely rotate and pivot relative to the screw. Once the screws are screwed into the corresponding vertebrae, the rod is positioned within the transverse passage of each tubular body. A nut is then screwed onto the exterior of the tubular body. The nut biases the rod against the head of the screw so as to rigidly secure the rod to the anchor.
Although spinal stabilizing systems are commonly used, conventional systems have a number of shortcomings. For example, mounting of the screw into the bone requires a thin elongated driver that mates with the polygonal socket on the head of the screw. Conventional drivers can be difficult and awkward to use resulting in misalignment of the screws. Furthermore, on occasion it is necessary to remove a screw after it has been implanted for an extended period of time. While the screw is implanted, however, tissue and/or bone typically grow over the head of the screw, thereby making it difficult to access the screw and couple the driver with the screw.
In addition, because the socket is formed on the top of the head of the screw, the top surface of the head is flat. During use, the rod rests on top of the head of the screw. However, the tubular body is often pivoted relative to the longitudinal axis of the screw so that the rod can be received within the transverse passage. As a result of the tubular body being pivoted, the rod often rests irregularly on the corner of the flat surface formed on the head of the screw. This irregular seating of the rod on the head of the screw can produce a weak connection, produce undesired pivoting of the screw or tubular body, and/or produce unwanted stress on the spine.
Furthermore, as the nut is tightened on the tubular body, the remainder of the anchor needs to be stabilized so that undue loads are not applied to the spine. To accomplish this, an anti-torque device is passed over the tubular body so as to engage only the rod passing therethrough. An opposing force is then applied by the anti-torque device to the rod as the nut is tightened onto the tubular body, thereby minimizing undue stress on the spine. One difficulty with this approach, however, is that the rod is often disposed directly on or adjacent to the bone and/or tissue. As such, it is often difficult and time consuming to adequately place the anti-torque device over the rod.
As mentioned above, a nut is screwed onto the exterior of the tubular body to bias the rod against the head of the screw so as to rigidly secure the rod to the anchor. This also simultaneously secures the tubular body to the screw. In other words, the collar is not secured to the screw until the rod is biased against the head of the screw, and in fact can rotate or pivot with respect to the screw. This points out another shortcoming with current systems. There are times a doctor wants to first secure the tubular body to the screw to form a rigid anchor, and then adjust the resulting rigid anchor with respect to the rod before the anchor is secured to the rod. For example, a doctor may desire to further separate or bring together vertebrae that have been crushed or otherwise affected. In most conventional systems this is not possible because, as pointed out above, the tubular body is not secured to the screw until the rod is secured using the nut. To solve this problem, included in some conventional systems is a separate set screw that allows the doctor to secure the tubular body to the screw independent of the rod being secured to the screw. While this helps solve the problem, having two screws instead of one makes the system more complex and exacerbates the problems mentioned above relating to drivers and unwanted torque being applied to the spine.
Accordingly, it would be beneficial to have spinal stabilizing systems that address some or all of the foregoing shortcomings.